The present invention relates to fuel cell stacks that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fuel cell stack having thin and thick fuel cells arranged so that between two and five thin fuel cells are secured adjacent each other and adjacent each thick fuel cell to reduce an overall volume of the stack.
Fuel cell power plants are well known and are commonly used to produce electrical current from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as power plants and transportation vehicles. In fuel cell power plants of the prior art, it is known that water management and cooling tasks are frequently managed by a cooling system. As disclosed in U.S. Pat. No. 5,503,944 that issued to Meyer et al. on Apr. 2, 1996, and in U.S. Pat. No. 6,521,367 that issued on Feb. 18, 2003 to Reiser, which patents are owned by the owner of all rights in the present invention, such cooling systems typically include a porous water transport plate defining channels to direct water from and through the fuel cell.
Product water generated during operation of the fuel cell is frequently directed as liquid water into the porous water transport plates and out of the fuel cell within the channels of the plates, as well as by being entrained as water droplets and water vapor within exhaust streams of fuel cell reactants within reactant flow fields that are also often partially defined within the water transport plates. Additionally, cooling water is directed to flow through the channels of the water transport plates through the fuel cells to remove heat generated during operation of the fuel cells. If the water transport plates are porous, such cooling water also moves through the water transport plates into adjacent reactant flow fields to humidify reactant streams and prevent dry-out of proton exchange membrane (xe2x80x9cPEMxe2x80x9d) electrolytes.
Consequently, it can be seen that known porous water transport plates facilitate both water management and cooling functions. Water management functions include humidification of reactant streams and removal of liquid condensates and fuel cell product water. Cooling functions include removing heat generated during operation of fuel cells. It is also well known that a plurality of fuel cells are typically disposed cooperatively to form a fuel cell stack including manifolds and headers to deliver and remove reactant and coolant streams. In manufacture of such fuel cell stacks, it has become common for purposes of manufacturing efficiency to utilize a water transport plate defining channels for directing water from and through the fuel cells of the stack that accomplish both the water management and cooling tasks.
However, as such fuel cell stacks become useful in powering transportation vehicles, such as busses, trucks automobiles, etc., it is imperative that the fuel cell stacks occupy as small a volume as possible and have the lowest possible weight. Additionally, such transportation vehicles are typically exposed to sub-freezing ambient conditions below the freezing temperature of water. Therefore to enhance efficiency of the stack upon start up in such sub-freezing conditions, it is necessary that the fuel cell stack contain the least possible volume of water.
Consequently, there is a need for a fuel cell stack with the least possible overall volume and weight, and with the least possible volume of water.
The invention is a fuel cell stack for generating electrical current that has a reduced volume compared to known fuel cell stacks. The fuel cell stack includes a plurality of thin fuel cells and a plurality of thick fuel cells. The thin fuel cells include a water management water transport plate in fluid communication with a reactant flow field of the thin fuel cell. The water management water transport plate defines water management channels for directing flow of water from and through the thin fuel cell to facilitate water management functions of the fuel cell stack including humidification of reactant streams and removal of condensate and fuel cell product water.
The thick fuel cells are cooperatively disposed with the thin fuel cells to form the fuel cell stack along with known manifolds and headers, etc. Each thick fuel cell includes a combined water management and coolant water transport plate in fluid communication with a reactant flow field of the thick fuel cell. The combined water management and coolant water transport plate defines cooling channels for directing flow of water from and through the thick fuel cell. The thick fuel cells are secured within the fuel cell stack so that at least two thin fuel cells are secured adjacent each other and adjacent each thick fuel cell. Also, the water management channels defined within the water management water transport plates of the thin fuel cells have a depth parallel to a longitudinal axis of the stack that is at least four times less than a depth of the cooling channels defined within the combined water management and coolant water transport plates of the thick fuel cells. The longitudinal axis of the fuel cell stack runs through the stack and is also a shortest distance between non-adjacent fuel cells of the stack.
Water management functions of humidification and removal of condensed water from reactant streams and removal of fuel cell product water require only a small fraction of the water flow through each fuel cell that is required for cooling a fuel cell. However, cooling can be effectively achieved for between about two to five fuel cells by passing an adequate volume of cooling water through the combined water management and coolant water transport plate of one thick fuel cell.
Therefore, by having one thick fuel cell for every two to five thin fuel cells, the thinner water management water transport plates of the thin fuel cells provide for a significant reduction in both an overall volume and weight of the fuel cell stack and also in an overall amount of water resident within the fuel cell stack during operation of the stack. Decreasing the amount of water in the fuel cell stack facilitates start up of the fuel cell stack in sub-freezing ambient conditions.
Accordingly, it is a general purpose of the present invention to provide a fuel cell stack having a reduced volume that overcomes deficiencies of the prior art.
It is a more specific purpose to provide a fuel cell stack that reduces an overall volume and weight of the fuel cell stack.
It is yet another, purpose to provide a fuel cell stack that has the least possible volume of water.
These and other purposes and advantages of the present fuel cell stack having a reduced volume will become more readily apparent when the following description is read in conjunction with the accompanying drawings.